Measuring deviations from a perfectly circular cross-section of an optical nanofiber at the \r{A}ngstr\"om scale

TL;DR

This paper investigates Angstrom-scale deviations in optical nanofibers' cross-section, revealing that they are nearly elliptical with minimal asymmetry, which affects polarization stability crucial for quantum and nanophotonic applications.

Contribution

It introduces two in-situ methods to measure nanofiber cross-section deviations at the Angstrom scale, providing detailed characterization of their elliptical shape and polarization effects.

Findings

Nanofibers have an elliptical cross-section with a radius of ~255.6 nm and ~2 Å difference in semi-axes.

Polarization changes periodically along the nanofiber due to birefringence from the elliptical shape.

The two measurement methods agree on the elliptical shape and orientation of the polarization eigenaxes.

Abstract

Tapered optical fibers (TOFs) with sub-wavelength-diameter waists, known as optical nanofibers, are powerful tools for interfacing quantum emitters and nanophotonics. These applications demand stable polarization of the fiber-guided light field. However, the linear birefringence resulting from \r{A}ngstr\"om-scale deviations in the nanofiber's ideally circular cross-section can lead to significant polarization changes within millimeters of light propagation. Here, we experimentally investigate such deviations using two in-situ approaches. First, we measure the resonance frequencies of hundreds of flexural modes along the nanofiber, which exhibit splitting due to the non-circular cross section. By analyzing the mean resonance frequencies of each pair and the corresponding frequency splitting, we conclude that the nanofiber can be well described as having an elliptical cross-section with a mean radius of 255.6(9) nm, where the semi-axes differ by only about 2\r{A}. Second, we monitor the polarization of the guided light field by imaging the light scattered out of the nanofiber and observe a periodic polarization change along it. From the linear birefringence due to the elliptical cross-section, we infer a comparable difference in the semi-axes as the first method, and determine the orientation of the polarization eigenaxes. Our work is crucial for any fundamental or applied study that requires a well-controlled interaction between guided light and matter, in particular for quantum memories, frequency conversion, or lasing that require a large interaction length.